1. Field of the Invention
The invention relates to the production of perchloroethylene and trichloroethylene by catalytic oxychlorination of C.sub.2 hydrocarbons or their partially chlorinated derivatives, to catalyst compositions for use in such oxychlorination processes and to modified alumina supports used in making such catalysts.
2. General Background and Summary of the Prior Art
Perchloroethylene, C.sub.2 C.sub.14, and trichloroethylene, C.sub.2 HCl.sub.3, are chlorinated hydrocarbons which are widely used as solvents in dry cleaning textiles, in degreasing metal parts, in various solvent extraction processes, in compounding rubber cements, and in various other operations. Because perchloroethylene is relatively stable, its use is much less severely restricted by anti-pollution regulations than is the use of most other chlorinated solvents and is therefore a particularly desirable product. Trichloroethylene is becoming increasingly important as a raw material for the manufacture of replacement refrigerants for fully halogenated refrigerant compounds currently in use.
Both C.sub.2 Cl.sub.4 and C.sub.2 HCl.sub. have been commonly produced by catalytic oxychlorination of ethane, ethylene or a partially chlorinated derivative thereof, i.e., by reacting such a feedstock with hydrogen chloride or chlorine and air or oxygen at a suitable temperature in the presence of a suitable catalyst which is maintained in the reaction zone either as a fixed bed or, more preferably, as a fluidized bed.
Typically, such catalyst compositions comprise a catalytic amount of a metal having a variable valence, such as copper, as well as an alkali metal, such as potassium, supported on a suitable carrier Carriers used commercially in the past have included highly calcined fuller's earth, such as "Florex", or preferably synthetic activated aluminas. See, for instance, U.S. Pat. Nos. 3,267,162, 3,296,319 and 4,463,200. Fuller's earth is essentially a magnesium-aluminum silicate containing small proportions of oxides of iron, calcium, potassium and titanium. By contrast, synthetic activated alumina consists essentially of alumina with virtually no significant impurities or at the most only a very small proportion o silica.
Researchers working on catalytic oxychlorination processes in the past have in some instances expressed a preference for the use of low-surface area alumina as catalyst supports, i.e., for supports having a surface area below 10 m.sup.2 /g, and especially between 2 and 5 m.sup.2 /g, as in U.S. Pat. No. 4,124,534. In other instances they have expressed a preference for high-surface area alumina as catalyst supports, i.e., for supports having a surface area of at least 100 m.sup.2 /g, as in U.S. Pat. No. 4,463,200. Low-surface area supports have been recommended mainly because they were thought to result in higher HCl conversions and lower carbon burning; see, for instance, U.S. Pat. No. 3,427,359 and French Patent No. 1,386,023. On the other hand, high-surface area supports have been recommended because they were thought to contribute to an increased selectivity of the reaction toward the production of perchloroethylene as the desired product and a reduced formation of undesirable 1,1,2-trichloroethane and unsymmetrical tetrachloroethane, as indicated in U.S. Pat. No. 4,463,200.
Catalysts comprising a low-surface area support have been found to be relatively unstable in that such supports possess only a relatively small number of binding sites for retaining the active metal salts in the composition, and the resulting loss of the metal salts from such catalysts has been found to constitute a significant factor in causing corrosion of the metal reactors in which such oxychlorination reactions are generally carried out. In addition, especially when low-surface area supports such as diatomaceous earth or other silica-aluminas are used, high selectivities to perchloroethylene and trichloroethylene are only obtained at very high temperatures.
Catalysts based on high-surface area supports, which have been used successfully in producing ethylene dichloride at temperatures below 350.degree. C., retain their catalytic salts well but have been found to cause substantial destruction of the feed material because of its oxidation to form carbon oxides. This becomes especially serious when unchlorinated ethane or ethylene or ethyl chloride is used as the feed to make more highly chlorinated hydrocarbons such as perchloroethylene and trichloroethylene, the production of which requires reaction temperatures near or above 400.degree. C. in order to obtain good selectivity. Conversely, poor selectivity to the desired products has tended to occur when low-surface area aluminas were used as catalyst supports. Moreover, such prior catalyst compositions have been frequently found to have a relatively limited useful life because they have only moderate thermal stability and consequently tend to lose surface area and become sticky and corrosive to metal reactor walls as the oxychlorination process continues, especially at reaction temperatures above 350.degree. C.